of the New Red Sandstone of the Mendip Hills. 423 



The more perfect agates I am now considering are found in the dolomitic 

 beds of the new red sandstone formation. The spot on which I saw them, 

 lies between the villages of Banwell and Churchill, at the north base of a hill 

 called Sandford Hill;, where the junction of the wood with the cultivated land 

 marks the geological junction of the inclined mountain limestone with the 

 horizontal beds of dolomite. Here a shallow pit, which has been opened for 

 the express purpose of digging agates, presents the following section : 



1. Yellow clay, mixed with carbonate of magnesia and carbonate of lime 6 inches. 



2. Yellow dolomite, used as firestone in lime kilns ; it crumbles readily to a soft T 



powder, and is filled with specks of manganese, and contains small veins and V G inches, 

 minute nodules of chalcedony J 



3. Yellow clay, falling to powder in water, like fuller's earth, and containing much "| 



carbonate of lime and magnesia. In this clay the agates are dispersed irre- |- 6 inches, 

 gularly, like nodules of flint in chalk, but not like them in horizontal lines. . J 



4. Yellow clay and earthy dolomite, to the bottom of the pit 12 inches. 



In the adjacent field is an open well, about twelve feet deep, showing the 

 continuation downwards of the same argillaceous, earthy dolomite which forms 

 the bottom of the agate pit. 



These beds of dolomitic clay seem to be decomposed strata of dolomite, in 

 the cavities of which, before its decomposition, the agates may have been 

 formed. The soft, arenaceous, yellow dolomite. No. 2., would, by a very little 

 decomposition, be reduced to a state much resembli ig the yellow clay of 

 No. 3. In the more solid and crystalline slabs of the stratum No. 2. I found 

 siliceous concretions, which, on being broken, proved to be coarse potatoe- 

 stones ; whilst the softer and yellow portions of the same stratum contained 

 thin veins of opaque and white chalcedony, and minute insulated nodules of 

 agate. The substance of these veins and agates is precisely like that of the 

 large agates in the clay immediately subjacent. 



This union of potatoe-stones with veins of chalcedony, and with small 

 agates, in the solid dolomite immediately reposing on the clay containing the 

 larger agates, shows that their common origin may be due to an infiltration 

 of siliceous matter into cavities. I do not, however, contend that all these 

 agates and potatoe-shaped concretions have been formed by infiltration into 

 cavities of consolidated rocks : some may have been formed contempora- 

 neously, whilst their matrix was yet plastic, and unconsolidated, and admitted 

 the separation of its siliceous from its calcareous ingredients, in the same 

 manner as contemporaneous concretions of flint have been formed in chalk, 

 and nodules of chert in various limestones, and as Septaria have been formed 

 in beds of clay. 



3 I 2 



